apic: initialize lapic_paddr statically

The default value for LAPIC registers page physical address
is usually right. Having this value available early makes
pmap_force_invalidate_cache_range(), used on non-self-snoop machines,
avoid flushing LAPIC range for early calls.

Reviewed by: alc, markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D32318